In mining coal underground in low coal, where the seams, typically, are 2.5-4 feet thick, the room and pillar method of mining often is used. Referring to FIG. 1, which is a fragmentary horizontal sectional view at one level in such a mine, a typical layout is illustrated. A main gallery is illustrated at 10. Shown branching out laterally from the main gallery are a plurality of rooms 12, 14, which are separated by pillars 16. Each pillar 16 is an expanse of coal left unmined in order to retar or prevent subsidence. Although in such a mine, mining may simultaneously be going on in several of the rooms and at several levels, for simplicity a single room 14 is shown having mining going on in it. In brief, a mining machine makes a series of cuts into the face or short wall 18, as a result of which the room 14 gradually becomes longer in the direction of the arrow 20.
It is well-known that as coal is mined, dust and gases may be liberated, especially at the face 18, which gases or dust if merely left to accumulate, could build-up to explosive or unhealthful concentrations. Accordingly, there has long-ago been devised a system for ventilating such rooms while mining is going on, in order to pull so much air across the face that potentially noxious concentrations of escaping gases or dust cannot build-up, because whatever is evolving is whisked-away so quickly. In practicing the conventional ventilating system, the room 14 is nearly divided by a longitudinal line curtain or brattice 22 which extends from the main gallery 10 to a point near the face 18, and from the floor to the roof. Typically, the brattice is made of a fire-proofed canvas that is fastened to a batten that is bolted to the roof and/or fastened to posts and it hangs to the floor, where the excess is weighted-down with rocks. Usually, the brattice is not placed on the longitudinal center line of the room, but considerably off toward one side so the room is divided into a wider portion 24 and a narrower portion 26. By means of fans and the like, a pattern of circulation is set up in the direction of the arrows 28. This pattern carries fresh air in from the main gallery 10 through the wider portion 24, sweeps that air across the face 18 and back out through the narrower portion 26. In order to maintain this pattern, the brattice must have its leading edge 30 near the face, but spaced somewhat away from it.
As the room 14 is lengthened in the direction of the arrow 20 through the efforts of the miner operating the mining machine 32 periodically the workers have to stop mining and work on making the curtain extend further, by erecting poles and nailing more curtain material to them, etc.
The problem is that the U.S. government does not want any workers to be right up near the face that is being mined until after the roof has been bolted by a cage-protected roof bolting machine operator (not shown).
So there is somewhat of a conflict in the present regulations: how to extend the curtain, at least temporarily, without any person going into the region where the new section of curtain must be installed.
There are disclosed in the prior art some ways for temporarily extending the line curtain toward the face that is being mined as that face recedes, until another section of regular line curtain can be installed, but none of the prior art ways are entirely satisfactory. In one prior art technique, a support bar is cantilevered horizontally forwards from the top of the foremost support post of the regular line curtain. A section of curtain is hung from this support bar and has its leading edge periodically pushed forwards. Toward the extreme of advancement, the cantilevered support tends to droop, and the lower forward corner portion of the curtain may flap in the currents of ventilating air and tend to close-off the narrow section of the room; or direct the air through the gap between the curtain and the roof and not direct the air across the coal face.
In other prior art attempts at solving the problem, the mining machine itself has been provided with a movable line curtain extension. The result is a compromise: the machine is less maneuverable and the curtain extension is not as effective as one that is mounted independently of the mining machine.